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dc motor speed control Answered

i need help regulating the speed of a simple dc hobby motor. i understand that i should be able to do this with a variable resistor/potentiometer but while it does work, it only does so when the wiper gets really close to one end. how do i even it out? i also must admit i don't understand complex electronics and wish to find a simple solution. any help is much appreciated.



11 years ago

To do it with a pot, you need the resistance of the pot to be similar to the resistance of the motor, which is usually small numbers of ohms, so you'd need something like a 20ohm pot (maybe a speaker "fader" control?) I've been meaning to do an instructable on using an LM317 (variable voltage regulator) as a simple speed control; you could set it up with a max output voltage the same as your power supply, and it would provide up to an amp, with assorted protections, while removing any serious demands on the pot itself, and allowing for varying pot values by changing the fixed resistor. But if you wait for me to get around to it, you may be waiting a long time.

> maybe a speaker "fader" control? . Which would better, log or linear taper? . Is a "fader" different than a volume pot? . It's been a few decades since I did any breadboarding or audio work. :)

For a motor control, you probably want a linear pot. I suspect most audio fader pots are log :-( The "speaker fader" that I'm trying to describe is a volume control that goes in between the speaker outputs of your amp and the action (8-ohm) speaker. So they're designed to handle the power and voltage the speaker will see, which is significant and comparable to small motors. A "volume pot" could be any pot anywhere in a circuit, so it's PROBABLY different. Tool Using Animal's PWM kit looks like a pretty good deal...

. Thanks for responding. . When I say "audio/volume pot," I'm thinking of the amplifier's gain/feedback control (voltage divider). . IIRC, the generic name for a "speaker fader" is rheostat (designed to control current). I think there is a special name name for "audio rheostats," but I can't recall what it might be. (When I see "fader" I visualize the left/right/front/rear controls.) . Would there be any advantage to using PWM over, say, a switching power supply for hobby DC motor speed control?

PWM and a (variable) switching power supply should be able the same, and much better than the inefficient linear 317 I was suggesting. In fact, I consider PWM (as done for motor control) to be a TYPE of switching power supply (when the load doesn't really require DC.) PWM is theoretically much simpler and is efficient over a wide range of output power, but with single-chip switching power supply chips using relatively few add-on components, it's hard to say which one is simpler In Real Life.

> I consider PWM (as done for motor control) to be a TYPE of switching power supply . I thought about that after I posted. It was just an example that I pulled off the top of my head. I should have said "any other type of PS." Lucky for me, you gave enough info that it doesn't matter.

> But if you wait for me to get around to it, you may be waiting a long time. . Everytime I get a round tuette, I just throw it in the back of a kitchen drawer. I have quite a collection. :)

. Try using a "smaller" (lower Ohms) pot. . You may have an audio (log) taper pot. Try a linear taper pot.